A new adaptive beamforming method for accurately focusing ultrasound behind highly scattering layers of human skull
and its application to 3D transcranial imaging via small-aperture planar phased arrays are reported. Due to its undulating,
inhomogeneous, porous, and highly attenuative structure, human skull bone severely distorts ultrasonic beams produced
by conventional focusing methods in both imaging and therapeutic applications. Strong acoustical mismatch between the
skull and brain tissues, in addition to the skull's undulating topology across the active area of a planar ultrasonic probe,
could cause multiple reflections and unpredictable refraction during beamforming and imaging processes. Such effects
could significantly deflect the probe's beam from the intended focal point. Presented here is a theoretical basis and
simulation results of an adaptive beamforming method that compensates for the latter effects in transmission mode,
accompanied by experimental verification.
The probe is a custom-designed 2 MHz, 256-element matrix array with 0.45 mm element size and 0.1mm kerf. Through
its small footprint, it is possible to accurately measure the profile of the skull segment in contact with the probe and feed
the results into our ray tracing program. The latter calculates the new time delay patterns adapted to the geometrical and
acoustical properties of the skull phantom segment in contact with the probe. The time delay patterns correct for the
refraction at the skull-brain boundary and bring the distorted beam back to its intended focus. The algorithms were
implemented on the ultrasound open-platform ULA-OP (developed at the University of Florence).